Cell Membranes:
Essential Concepts

Membranes are assemblies of carbohydrates, proteins,
and lipids held together by non-covalent forces. They regulate
the transport of molecules, control information flow between cells,
generate signals to alter cell behavior, contain molecules responsible
for cell adhesion in the
formation of tissues, and can separate charged molecules for cell signaling
and energy generation.

Cell membranes are dynamic, constantly being formed and degraded.
Membrane vesicles move between cell organelles and the cell surface.
Inability to degrade membrane components can lead to lysosomal storage
diseases.

Lipids of cell membranes include phospholipids composed of
glycerol, fatty acids, phosphate, and a hydrophobic organic derivative
such as choline or phosphoinositol. Cholesterol is a lipid component
of cell membranes that regulates membrane fluidity and is a part of
membrane signaling systems. The lipids of membranes create a hydrophobic
barrier between aqueous compartments of a cell. The major structure
of the lipid portion of the membrane is a lipid bilayer with hydrophobic
cores made up predominately of fatty acid chains, and hydrophilic surfaces.

Membrane proteins determine functions of cell membranes, including
serving as pumps, gates, receptors, cell adhesion molecules, energy
transducers, and enzymes. Peripheral membrane proteins are associated
with the surfaces of membranes while integral membrane proteins are
embedded in the membrane and may pass through the lipid bilayer one
or more times.

Carbohydrates covalently linked to proteins (glycoproteins)
or lipids (glycolipids) are also a part of cell membranes, and function
as adhesion and address loci for cells.

The Fluid
Mosaic Model describes membranes as a fluid lipid bilayer with
floating proteins and carbohydrates.

Cell junctions are a special set of proteins that anchor cells
together (desmosomes), occlude water passing between cells (tight junctions),
and allow cell to cell direct communication (gap junctions).